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Flowering plants grow through somatic cell division, and leave their offspring through reproduction, which is an important lifetime. Plant reproduction involves gametophyte formation, fertilization, and embryogenesis. Female gametophyte (FG) is crucial for reproduction in flowering plants. Mutational analyses using Arabidopsis thaliana have identified genes and proteins involved in FG development. Techniques that induce gene expression by different external factors have been used as powerful tools for the analyses of various gene functions in plants. In this study, I developed a new method to analyze gene functions during female gametophyte development using heat-inducible Cre/loxP recombination system and FG-specific EMBRYO SAC 2 (ES2) promoter. Gene expression from the ES2 promoter can be detected from the four-nucleated FG stage. Efficient gene induction can be achieved by mild heat treatment at 35°C for 5 min. I applied this method to study gene functions in the fusion of polar nuclei during FG development. Sad1/UNC84 (SUN) proteins are conserved inner nuclear membrane proteins, which interact with the Klarsicht/ANC-1/Syne homology (KASH) proteins spanning the outer nuclear membrane. A. thaliana has five SUN protein genes. I used this system to express a dominant-negative mutant of SUN proteins (SUNDN) in developing FGs. Expression of SUNDN resulted in inhibition of the polar nuclear fusion, while expression of a SUNDN mutant, which does not interact with KASH proteins, did not cause the polar nuclear fusion defect. These results suggest roles of SUN-KASH interactions in the fusion of polar nuclei. The heat-inducible gene expression system developed in this study provides a new strategy for analyzing gene functions in FG development and FG functions., 新大院博(理)甲第451号